SINAMICS S120 Position Correction: Resolving 5mm Z-Axis Error

Problem Description: Persistent 5 mm Position Deviation on a CNC Z-Axis Driven by SINAMICS S120

A retrofit CNC milling machine equipped with a SINAMICS S120 drive on the Z-axis reports a repeatable position deviation of 5 mm. The operator enters a target of 980 mm on the HMI/operating panel; when the axis stops, a hand-measurement with a ruler shows 975 mm. The error is consistent in sign and magnitude across multiple moves, which rules out random mechanical slippage but points to a systematic scaling, tuning, or feedback issue inside the SINAMICS S120 closed-loop chain.

Because the SINAMICS S120 position loop is only one half of a typical CNC motion chain (the upper CNC controller closes position, the drive closes current and speed), a 5 mm steady-state deviation can be introduced by any of the following: an incorrect Length Unit (LU) scaling, an un-tuned speed controller, missing speed precontrol, wrong encoder pulses per revolution, missing backlash compensation, or a mismatch between CNC and drive position reference. This reference walks through every parameter in the chain that has to be verified.

Closed-Loop Architecture: Where the Position Error Is Generated

In a CNC–SINAMICS S120 topology the position loop is closed by the CNC, the speed loop is closed by the drive, and the current loop is closed by the drive. The SINAMICS S120 still contains a position controller that can be used as a secondary fine-positioning loop or for stand-alone positioning. The standard position-control signal chain is:

1. CNC position setpoint → PROFIdrive telegram (telegram 110 or 111, NSOLL/NSOLL_B) → drive word.
2. SINAMICS position controller (p2500…p2534) generates a speed setpoint from the position error (r2521 = position actual value in LU).
3. Speed precontrol (p2534, %) adds a feed-forward term so the speed controller does not have to integrate the entire position error.
4. Speed controller (p1460, p1462) generates a torque setpoint from the speed error.
5. Current controller (p1715, p1717) closes the torque loop on the motor.

The SINAMICS-internal calculation of speed from the position difference is described in detail in the SINAMICS S120 Function Manual Safety Integrated (FH1, section "Position and speed calculation"): Pos diff = Pos new − Pos old; the speed is then derived as Pos diff / T_s. If the LU per load revolution is wrong, both the actual position and the internal speed are wrong by the same factor, which produces a steady-state offset rather than a dynamic following error.

Root-Cause Matrix for a Steady-State 5 mm Z-Axis Offset

Step 1 — Verify Position Scaling (LU per Load Revolution)

The Length Unit (LU) is the user-defined resolution of the position controller; it determines how many LU correspond to one load revolution and how many LU correspond to one motor revolution. A 5 mm offset that does not scale with travel distance almost always indicates a wrong LU per load revolution (p2506).

Scaling calculation

For a ball-screw with pitch P (mm/rev) and a reduction ratio i = n_motor / n_load:

p2506 [LU/rev] = P [mm/rev] × 1000 / R [µm/LU]

With P = 10 mm/rev, R = 1 µm/LU → p2506 = 10 000 LU/rev. If p2506 is set to 9 500 the drive will think a 10 mm mechanical move is only 9.5 mm, producing exactly the 5 ‰ (0.5 %) error class observed on a 980 mm target (980 × 5 ‰ = 4.9 mm ≈ 5 mm).

Step 2 — Enable and Tune Speed Precontrol (p2534)

Even with perfect scaling, an un-tuned speed controller can leave a residual position error if the I-term of the speed controller never has time to wind up. Speed precontrol injects the demanded velocity directly into the torque setpoint so the speed controller only has to correct disturbances.

1. Set p2534 = 100 % (full precontrol).
2. Set p2535 = 0 (precontrol from setpoint, not filtered).
3. Set p2536 = 0 (no speed precontrol of torque setpoint — leave at 0 unless using a direct setpoint).
4. Re-run the auto-tuning wizard (STARTER / Startdrive → Commissioning → Speed controller).

With precontrol active the position controller output (r2526) is no longer the only feed to the speed controller; the velocity feed-forward term (r2527 / p2534) covers the bulk of the speed setpoint, and the I-term of the speed controller only has to compensate friction and load torque.

Step 3 — Optimize the Speed Controller (p1460, p1462, p1715)

Open STARTER / Startdrive and capture a trace of r0061 (speed actual) and r0063 (torque setpoint) during a representative Z-axis move. Use the one-button tuning function:

1. Set p1910 = 1 (motor identification) — run with the motor uncoupled if possible.
2. Set p1960 = 1 (speed controller optimization with rotation).
3. Accept the proposed Kp (p1460) and Tn (p1462) values.
4. Set the current setpoint filter p1715 to the suggested value (typically 0.5 × 1/(2π×Kp)).

If the optimization rejects a stable result, manually de-tune by halving Kp and doubling Tn, then re-test. A sign-inverted Kp (negative) will drive the axis away from the setpoint and is the most common cause of a growing rather than constant error.

Step 4 — Verify the Encoder (p0408, p0421, p0431)

For a 5 mm constant offset, the encoder pulse count is rarely the cause, but a wrong PPR will scale the error with distance. Confirm:

Read r0061 during a 60 rpm reference move; if the drive shows exactly 60 rpm, the encoder is wired and parameterized correctly. Read r0979[0…30] to capture the encoder identification; cross-check the listed PPR against the nameplate.

Step 5 — Compensate Mechanical Backlash (p2583, p2584)

Backlash compensation only corrects the error produced by reversing direction. It does not correct a one-direction constant offset, but the two effects are often confused. Enable backlash compensation only after scaling is correct:

1. Measure the backlash: command +1 mm, read r2521; command −1 mm, read r2521; the difference is the mechanical backlash B (in LU).
2. Set p2583 = B (backlash compensation value).
3. Set p2584 = 1 (backlash compensation active).
4. Re-run the 980 mm move in both directions; the offset should be within ±1 LU.

Step 6 — Synchronize CNC Machine Data with the Drive

On a CNC retrofit the drive may be correct and the CNC machine data (MD) may be the source of the mismatch. Verify on the controller side:

• Lead screw pitch (often MD 30300 → 31030 family on Siemens SINUMERIK).
• Encoder pulses per revolution (MD 31020 / 31025).
• Gear ratio (MD 31060 / 31070 / 31080).
• Feed-forward gain (MD 32610 / 32810).

Command a small move from the CNC in JOG mode and read both r2521 (drive) and the CNC actual-value display. If the drive shows 10 000 LU and the CNC shows 9 500 µm, the CNC scaling is wrong, not the drive.

Verification Procedure After Correction

1. Power-cycle the drive (p0970 = 1) so all parameters are saved to the CF card.
2. Run a reference move to the machine zero and re-define the home position via p2596 / p2507 referencing mode.
3. Command moves of 10 mm, 100 mm, 500 mm, and 980 mm in both directions. Record r2521, r0061, and r0063 with a STARTER trace.
4. Accept the result when the difference between the CNC display and the mechanical measurement is ≤ ±1 LU (= 1 µm) at all four positions, in both directions.
5. Back up the parameter set with p0971 = 1 to the CF card / non-volatile memory.

Field-Proven Cautions

• Do not change p2506 on the fly with the axis enabled — the LU counter can jump and cause a following-error fault (F07452) on Safety-Integrated machines.
• When retrofitting a Sinamics S120 into a machine that previously used an older drive, the encoder wiring order (A+/A−, B+/B−, R+/R−) is often inverted between DRIVE-CLiQ and TTL encoders. A reversed B channel shows up as negative position feedback, not as a 5 mm offset.
• If the safety functions are active (SS1, SLS, SDI), changes to p2506 may trigger a STOP reaction. Temporarily set p9501 / p9601 to bypass mode only if the machine is in a controlled commissioning state; never run production with safety bypassed.
• For absolute encoders, an uncorrected homing (p2596 = 0) is the most common cause of a constant offset. Always re-home after changing p2506 or p0408.

When to Open a Siemens Service Request

If scaling, precontrol, encoder, and backlash have all been verified and a residual deviation of more than ±2 LU remains, escalate via the Siemens Industry Online Support Service Request tool. Attach the STARTER project archive, the function diagrams (FP2500, FP2520, FP2700), and the trace recordings requested above. A Safety-Integrated problem (F01611, F07452) should always go through a Service Request, not a forum thread.

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